


The Aquatic Equation

by madrabbitgirl



Category: Sherlock (TV), Sherlock - Fandom, Sherlock Holmes & Related Fandoms
Genre: Alternate Canon, American AU, Canon Divergent, Fluff and Crack, Johnlock - Freeform, M/M, Mermaids, Merslash, Philadelphia AU, Probably slash, Seriously no plans for what comes next, United states AU, Work In Progress, but mermaids, mer!lock, philadelphia canon
Language: English
Status: In-Progress
Published: 2016-08-02
Updated: 2016-08-05
Packaged: 2018-07-28 23:56:51
Rating: Not Rated
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 4
Words: 4,265
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7662163
Author URL: https://archiveofourown.org/users/madrabbitgirl/pseuds/madrabbitgirl
Summary: <blockquote class="userstuff">
              <p>When an experiment goes wrong and leaves Sherlock changed forever, he has to try and hide his secret from his brother, the police and his best friend. While John might not be the genius detective in the relationship, he's never been one to ignore a puzzle where Sherlock's behavior is concerned and makes it his new mission to find out what the man is hiding. Problem is, he's out of his depth.</p>
            </blockquote>





	1. Chapter 1

**Author's Note:**

  * For [MadMags](https://archiveofourown.org/users/MadMags/gifts).



> Hi! 
> 
> Not exactly Beta-ed. I like to write American Sherlock & John because I'm not British, so almost all of my stories are based in Philadelphia, PA.

The house had been silent until the tinkling of broken glass rang out, followed quickly by a rare and quietly uttered, "Damn."

There was more quiet while the person speaking tried to scrub up the spill, lips turned down in a disappointed frown. There was another 'damn', followed by a panicked 'fuck!' and then the sounds of his bare feet running to the bathroom. Each step was a slap of flesh on hard wood. He closed the door behind him. He didn't believe in gods, but if he did there would've been several thanks to them that the chemical spilled had only gotten on his skin, and only in a small quantity so his flatmate wouldn't be at risk. Still, there was no telling what would happen with this experiment.

The rash forming on his hands was more than a little disconcerting.

He looked in the mirror and gulped, tamping down on his panic.

  
His core reflection, the face he'd grown accustomed to when he did look at himself, was the same. He saw his sculpted chocolate curls, his gray-green eyes that were so pale they startled most people, and his full, pouting lips. On the downside, there were some things that were not the same. The blue scales that were forming on his hands were also forming along his sharp cheekbones. He was finding it difficult to breathe, probably due to the openings just under his ears along his throat that resembled gills.

  
Impossible. This was ridiculously impossible. The chemical compound should not have absorbed that quickly and, even so, should not have produced these results. Instinct had him flicking on the taps, filling his tub with water. He hurried to shimmy out of his expensive trousers, leaving them in a puddle on the floor. This was not happening. His rational, logical mind was not going to accept that this was happening, even as his body changed before his very eyes. Worthy of yet another 'fuck', he slipped into the tub and submerged his head under water. He felt instantly better and at the same time much, much worse. A sickening nausea was spreading through him as he watched the scales grow, morphing the skin on his legs while they melted together and formed a strange, hypnotic tail. It slapped the tile flooring as he struggled. Questions swirled in his mind. What had the victim been working on? What exactly was the goal of this particular compound? Had the victim really gone missing or... had she simply had a similar accident?

  
Whatever was really going on, he couldn't involve the police any further or his own brother. In both instances he would end up locked in a lab somewhere, being the experiment instead of conducting them. No. There was no one he could tell.

  
Downstairs, the door opened and closed. His heart sank as every heavy footstep reminded him of the other person that lived in the house. The other person who could never, ever find out.

  
"Sherlock!" John's voice called through the house. Sherlock couldn't hear the exact words, but he heard the sound. Had he locked the bathroom door? Why didn't he remember?  
Was this that shock thing that people were always going on about?

  
"Sherlock, you home?" John shouted again. He had shopping bags in his hand but the mess on the table looked a little toxic so he went back and put them in the living room on the coffee table. He huffed in irritation about the mess. "Sherlock!"

  
"In here!" Sherlock called from the bathroom. He lifted his head just above water so he could hear John. Thankfully, from the way the lock was twisted, he could tell he locked the door. "Having a bath."

He could almost hear John frown from the other side of the wall. "A bath? You don't do baths, you said they were impractical and something about stewing in your own filth."

  
Sherlock's monstrous tail twitched, splashing water. "There was a bit of an accident with the last experiment and I need to soak. Don't touch the mess in the kitchen, I'll get it when I'm through."

  
"Definitely wasn't planning on it," John muttered under his breath. His hand touched the knob, although he didn't twist. "You sound strange, are you alright? Did you hurt yourself? I could-" Uncomfortable pause. "I could look at you, if you need me to."

  
"No!" Too enthusiastic. Sherlock needed to keep calm. "No, I'm fine, John, I assure you. Put the groceries in the refrigerator and go get carry out. No sense in us cooking until I've fully sterilized the kitchen, just to be sure. Go! Take my card!"

  
John hesitated. His gut twisted with instinct screaming that something was wrong with Sherlock. "Look, Sherlock, I know you're private but that's the same line of crap you gave me when you accidentally injected yourself with that new strain of Ebola-"

  
"Are you never going to forget that? John, I mean it, I'm fine," Sherlock insisted, but his voice sounded strained. He didn't know how to get out of the tub. He didn't know if his body would return to him- it seemed unlikely. Still, he couldn't stay in the bathroom forever.

  
John huffed, irritated and still worried. The sound of something heavy thudding to the floor on the other side of the door did nothing to make him feel better. "Sherlock-"  
"I'M FINE!" Sherlock shouted, patience wearing thin. He'd flopped onto the floor and managed to hurt his rear end in the process. He was having trouble breathing already. "Go, John. I'm fine."

  
"Great. Whatever. I'll be back in an hour and if you're not out of there, I'll break the damn door down," John returned with equal irritation. He stormed out the apartment only to have to immediately double back and put his groceries away before leaving again.

 

When John made his way back, Sherlock was sitting on the sofa clad in fresh PJ's and a soft tee shirt. His bony knees were drawn up tight against his chest. Strangely, despite having his lounge clothes on, he was also wearing a piece of outerwear- his blue scarf was wound around his throat. He didn't say anything as John placed the bag with their dinner in it on the coffee table.

  
"How was your soak?" John asked. Sherlock didn't say anything, he just shrugged. Sighing, John went into the kitchen in search of clean forks. The kitchen smelled like bleach and cleaning chemicals, but at least it was clean. Sterilized, as promised. "Thanks for cleaning up the mess, Sherlock. I appreciate it."

  
"Not a problem," Sherlock replied. Inside, John cringed. It wasn't like Sherlock to tidy fucking anything much less a whole room. Whatever he was working on had to have been pretty toxic for him to go to such extremes.

  
"Anything you want to talk about? Did your experiment get ruined?" John questioned, going back out into the living room.

  
Sherlock's eyes were on him in a way that made John uneasy. His flatmate was too tense, his posture too stiff, and his eyes held such intensity. John swallowed, but after a moment the look faded and Sherlock answered with, "Inconclusive results. I won't be repeating it."

  
"I guess that's something," John sighed. He swiped his food out of the bag, leaving Sherlock a fork, and settled down in his chair. "I'm starved. Anything else happen today? Did you ever figure out what happened with that missing lab assista-"

  
Before John could finish the sentence, Sherlock, his food and his fork were all gone and Sherlock's bedroom door was slammed shut.  
"Brilliant," John muttered to himself.


	2. Chapter 2

**Notes for the Chapter:**

> See notes in the beginning of Chapter 1 - like the one where I'm all 'hey I'm American'

John wasn't dumb.

He felt like he probably told himself that too many times to be healthy, but sometimes he needed to reassure himself. John was not dumb. Of course, compared to Sherlock Holmes, practically everyone was an idiot but John liked to think that he wasn't as moronic as everyone assumed. He couldn't put his finger on it, but he knew, somehow, that Sherlock was behaving a little... weird. Or, at the very least, weirder than usual.

He watched his room mate stumble around the scene, giving quick glances to the cloudy gray sky. He'd heard the weather report earlier. Sherlock watched it almost religiously now, which was a very new behavior. His deductions were rattled off but there was a hint of stress in his words. John looked up again. It hadn't been calling for rain today, just overcast, but the clouds looked heavy and grey and there was a hint of moisture in the air.

"Hey, is Sherlock alright?" Greg asked. He was standing next to John, shoulder to shoulder, but he leaned a closer to speak. "I hate to say it, but he's looking strung out."

John wanted to scold Greg for being a gossip, but he was inclined to agree. Sherlock was secretive, sly and enigmatic on a good day. Lately, though, his behavior had turned downright cagey. He jumped at any little disturbance, like he was waiting for Mycroft to appear at the top of the stairs and whisk him away. The blond doctor licked his lower lip thoughtfully. "He says he's fine."

"But you don't believe him," Greg pressed. His whisper turned into a hiss. "He's on one of my crime scenes, John-"

"Look, if he says nothing is going on, we have to believe him," John insisted. His stomach did nervous flip-flops as he doubted himself. "Not trusting him will get us no where."

"I've seen him look like that before," Greg muttered, not quite backing off.

"John, come. There's nothing for us here. Lestrade, have your people search the fields," Sherlock said suddenly. He started striding away at a fast pace. John cast an apologetic gaze at Greg before following. It really was a scenic little place for a murder- ancient cemetery, nice tree for the body to be hung from, lots of fields surrounding. The only shelter before them and their car was a tiny, one-room shack pretending to be a church.

"I don't think I'll make it," Sherlock muttered with another worried glance at the sky. He kept walking, but his stride sped up and John was practically jogging to follow.

"Look, you tall asshole, I can't-"

"Shut up, John. I'm trying to think," Sherlock snapped. It was at that point that the sky decided to open up, letting a heavy summer rain fall to the Earth below. Sherlock swore and started running for the church, John following close behind. He managed to slide in just before Sherlock slammed the door closed.

"No, no, no," Sherlock moaned, water dripping from his hair. He was starting to shimmy out of his trousers.

"Whoa! What are you-"

"No time, can't explain. Make sure the door is barred," Sherlock said, the familiar bluish rash creeping over his face. He barely got his pants off before he thudded to the floor, knocking the wind from him.

"It's a shack, Sherlock, it's not like-" John forgot what he was saying. He swallowed. Then swallowed again. He shifted uncomfortably. "Is this a joke?"

Sherlock's face, with his cheekbones edged in delicate blue scales, was murderous. He shivered with rage. His scarf had slipped away from his throat, revealing the gasping gills under his ears, and his legs had turned into a long tail. He was... beautiful. Unexpected, but gorgeous all the same. "Does it look like a joke to you? I just decide in the middle of working, John, to whip off my trousers-"

"So, not a joke, then," John said with a hysterical, high-pitched nervous giggle. "Not at all."

"No. Not a joke," Sherlock spat out. John just nodded.

And then promptly collapsed.

 

When John woke, they were in the car and driving on, what he assumed, was their way home. Farm fields were blending into suburban buildings, a sign that their city was just around the bend. He felt groggy and his head throbbed, which caused him to groan.

"John?" Sherlock's deep voice rumbled in the little car. "How are you feeling?"

John blinked. The last thing he'd remembered was Sherlock looking- well, like- but that wasn't- And now he looked normal again. His pale-pale cheekbones were without even a hint of blue, his scarf was tucked up tight around his throat, and he had legs. Good thing, too, since it would've been hard to drive without them. "Um. 'M fine. What-"

"You slipped," Sherlock filled in quickly. His eyes were trained on the road, with both hands on the wheel. It was a stiff position, and unnatural since Sherlock was actually a fairly good driver. He looked uncomfortable. "You hit your head. I was going to take you to the hospital on the way home."

The good doctor groaned, trying to process what he remembered. His head really did hurt. "But you- I thought I saw-"

"What did you see, John?" Sherlock asked lightly, still without looking at him. They were nearing the hospital, which John dearly wanted to avoid, but in light of what he'd... what he'd thought he'd seen?

"Nothing," John replied. Sherlock's jaw muscle twitched tensely. "No, I don't remember anything. Probably a good idea to get looked over, just in case."

"Quite," came the terse reply.

 


	3. Chapter 3

John couldn't help but be worried. Living with Sherlock could be challenging on a good day. After John's concussion, it was downright impossible. The sulky detective seemed to withdraw entirely. He was always locked in his room, the opposite of his usual couch-hogging behavior. He barely spoke to John or Mrs. Hudson. Then there was the scarf - he was never without it, even if he popped in the kitchen wearing his pyjamas. He also started taking long baths, which was strange, too. A few times John managed to sneak home early from work only to find Sherlock taking a long, indulgent soak in the tub.

Something was incredibly bizarre and try as he might, John just couldn't figure it out. He didn't believe Greg. Sherlock wasn't using drugs again. Maybe John was naive because every behavior indicated addiction. Moodiness, restlessness, excessively secretive- something was wrong and Sherlock wasn't letting John help him.

It really pissed him off, actually.

He mulled over these recent developments as he took his own after-work shower, washing the grime of the office away. It was altogether too easy to think of Sherlock in the shower. John had been doing it much too often lately. Leaning his head back against the cool tiles while warm water beat down on him. Gripping himself while Sherlock's fantastic face lingered in his mind-

With cheekbones rimmed in blue scales.

Groaning John switched off the taps, ready to bang his head against the wall. Something about that concussion-dream-hallucination was sticking with him and he found it hard to stop thinking of Sherlock as a- as a- mermaid?

John gave a hysterical laugh before he noticed something shining near the drain. When he bent to retrieve it, he realized what it was. A large, vibrant blue fish scale.

He gave a strangled noise, irritation bubbling in his veins. Wrapping a towel around his waist, he leaned out of the bathroom door. "Sherlock!"

There was a nervous clattering of china in the kitchen. "What?"

"Are you experimenting on fish in our bathtub?" John called. He stomped out to glare at his roommate, holding out the offending scale. "Did you seriously put fish in our bathtub? There isn't chemicals or something I need to be worried about, are there?"

Sherlock rolled his eyes like a petulant child being scolded by it's mother. "Where else would you like me to put them? You have odd reactions when I attempt to store experiments in the refrigerator."

"Just- Just promise me that I haven't just bathed in something toxic, alright?" John demanded, unable to admit that Sherlock had a tiny bit of a point. Sherlock's sea-colored eyes looked John over from head to toe. Blond hair dripping over his face, running in rivulets down his flat chest. John was taken in for a moment and he took a step forward. Sherlock countered with several steps backwards, plastering himself against the counter top.

It was a wordless rejection and it hit John too keenly. "Alright. Whatever. Have your experiments in the bathtub. I'm going out tonight, so just have it cleaned up before I get back."

John started back towards the shared bathroom where he left his clothes. He was surprised to hear Sherlock following, dragging his body against the wall as if to avoid where John had been walking. "John, where are you going?"

"Not that it's any of your business, but I've got a date tonight," John said through the closed bathroom door. He dressed quickly but neatly, a habit left from his military days. Clean jeans, his favorite sweater and nice shoes. He ran a comb through his hair, wondering if he should borrow Sherlock's blow dryer.

"Cancel it. It's pouring out," Sherlock demanded through the wooden panel. Ok, so no on the blow dryer.

"So?" John added deodorant and a tiny hint of cologne. "We're only going to the bar down the street. It's not far. I think I can handle a little rain, Sherlock."

He heard Sherlock heave a heavy sigh on the other side of the door. Opening it, he found the detective's face too close to his own. It would be so easy, with Sherlock's mouth already tipped down and John's chin angled up to close the distance and- and-

"I'll see you later, Sherlock," John said, swallowing and pushing past Sherlock. Sherlock followed, pausing at the door to the apartment.

"Have fun, John," Sherlock finally said, resigned to being alone during a rainstorm. "Don't get wet."

 

**Notes for the Chapter:**

> Still writing Americans because I'm still not British. LenaRidere did not Beta this but she did tell me it looked ok to put up. I blame all typos on here ;-P


	4. Chapter 4

The test of willpower had not gone smoothly. Results indicated that simply resisting the physical change did nothing to actually prevent it. He was beginning to feel a sense of desperation, of panic, setting in. John's current girlfriend was lingering and it set Sherlock even more on edge. She was an unpredictable variable in their lives. Girlfriends often were. John's routine schedule, except for when he was helping on a case, was work and then evenings in with the occasional night out with Lestrade. Experiments of a sensitive nature could be scheduled for when John was at the office, leaving their evenings free for take-out and puttering around the apartment. Now he could be going out to dinner and coming home or staying out all night or any number of strange things.

He'd better not bring that woman back to the apartment.

Now, with John not home at consistent times, Sherlock felt the urge to push his boundaries. To experiment in the evening as well. The danger of being potentially caught by anyone he knew (Lestrade, Mrs. Hudson, Mycroft or even John) added an addictive layer to his alarming yet thrilling situation.

He couldn't admit to himself that he water was... calling to him. That would be absolute madness. It would mean a lack of control, a lack of discipline, wouldn't it? To allow himself to believe some primitive, base instinct inside his newly changed DNA would be beckoned, as if by magic, to water? Ridiculous. No. He would not succumb to it so easily.

With John out for the evening, Sherlock did find himself giving into temptation. From what little he'd managed to gather, rain seemed to exacerbate his need to get in the water. Perhaps it was because of the moisture in the air. Data. He was missing so much data, and there was no reliable way of getting it without potentially revealing himself. He turned on the taps, letting the bathtub fill with water. He'd already discovered that the gills on his throat had no trouble breathing in fresh water or sea water, but there was a definite preference for sea so he added salts for comfort. It had taken a bit of experimentation to get the levels just right. As he lowered himself into the bath he tried once more to resist changing, but it did no good. Exactly thirty two seconds after the initial contact with liquid, his legs were bound together as a tail.

Detestable.

If only John were here. True, lately Sherlock had put distance between them, but even when John was home and Sherlock was sequestered in his bedroom at least there was another body in the house to make him feel less...alone. Not to mention drinks. John was fond of beverages- coffee before work, beer after dinner, water all the time. One of the first experiments Sherlock had conducted involved timing and contact with liquids. If any of those drinks happened to spill or had excessive condensation, Sherlock would have thirty two seconds to remove his trousers. He couldn't rely on John to faint every time he grew a tail. Eventually he'd have to admit what was happening.

No. Better to stay alone.

Sherlock contemplated his new, even lonelier existence. Before he'd only been separated from John by his friend's already flimsy heterosexuality and his constantly rotating catalog of girlfriends. Now there was more. Now there was a secret hovering over them, forever placing Sherlock in a category of his own. At least it was a category he was familiar with.

Freak.

His tail gave an unhappy flip. He wondered what a larger body of water would be like. He had to admit that the tub was beginning to feel... cramped.

A few hours later the door slammed downstairs. Sherlock's 'experiment' had devolved into an underwater nap but the uneven footsteps on the stairs had his eyes opening. John was not only home, but from the lumbering scuffle and scrape sound indicating a slight limp, he was drunk and upset. Sherlock's heart leaped.

He hadn't locked the bathroom door.

The risk of discovery was admittedly a little thrilling but he didn't want John to find him all the same.

"Sherlock! You!" John's voice called through the apartment. Ah. A break-up, then. Something had soured between the new lovebirds. John's steps came closer and the knob to the bathroom door twisted. "Sherlock?"

"No!" the detective yelped, splashing his tail. He wiggled unhappily in the tub. "No, I'm having a bath."

"I've seen men naked b'fore," John said cheerfully, hand still on the knob. "'M a doctor, y'know."

"Yes, yes, but don't come in," Sherlock snapped.

"Okay, okay," John groaned. From the dull swish and thump in the hallway, it sounded as though John had sat down just outside the door, bottom end on the wooden floorboards. "She dumped me, you know."

"So I gathered," Sherlock replied. His bathwater had gone cold but it didn't bother him as much as he would have expected. He'd need to conduct an experiment on the coldblooded nature of his... species. The thought that he might be something other than human twisted his stomach in knots.

"Is your fault she did that," John continued, his words mashing together. Sherlock popped the drain on the tub.

"My fault? How is it my fault?" Sherlock squawked indignantly. It couldn't possibly be his fault. In the past, he had spent many evenings interrupting, insulting or generally annoying the women John brought home in an attempt to make them leave, to let John continue with Sherlock just a little while longer as they were, but with this one he hadn't even tried. His own recent developments coupled with a month-long stretch of rain every day had kept Sherlock practically prisoner in his own home. He hadn't even tried to ruin John's chances of getting laid.

"Because you've been an ass," John told him. Sherlock heaved himself over the edge of the tub, dropping to the floor. "Lonely John. No friend, no girlfriend. She didn't like when I talked about you. No cases or adventures. She's boring."

Sherlock was not well acquainted with the finer points of human emotions, but something in John's words had him regretting his absence from his blogger's life lately. They solved cases together. They helped each other, but Sherlock just couldn't bring himself to ask for help with this. "You're upset with me."

"Yes! Ding ding! That one!" John said with a muffled giggle. "Final answer, Sherlock for the win."

"Because I've been distant with you?" Sherlock guessed. He frowned, beginning the tedious task of toweling off his muscular, scaly appendage.

"You make me sound like a teenage girl," John sighed. "No television, no take out, no cases. Not even experiments! Well, that's 'kay. No experiments doesn't suck. I like the fridge clean."

"I- I'm- Hmm." Sherlock, it seemed, was momentarily at a loss for words. Surely, John had enough to distract him? His now-ex girlfriend, work, drinking with Lestrade- wasn't there enough? It had never truly occurred to Sherlock that he might be missed. He leaned against the tub. The cold porcelain made him shiver. "I apologize, John. I'm working on an experiment that has taken up the majority of my time."

"It's okay. You're busy," John said. It would be so easy, especially with John drunk, to call him in. To show him exactly what had been going on. Perhaps it would take some of the stress out of the situation, being able to lean on someone else. In the end, John didn't give him a choice.

The doctor struggled to his feet, wavering a moment to bite back a sudden onslaught of nausea. "It's okay. Bed now, I think. Night, Sh'lock."

"Goodnight, John," Sherlock said, listening to the sound of John's retreating footsteps, waiting for his tail to dry.

 

**Notes for the Chapter:**

> See previous notes about 'original American canon ish' and 'not beta-ed' :-)


End file.
